1. Field of the Invention
The present invention relates to an apparatus for damping relative movement of two parts. In particular, the present invention relates to a fluid shock absorber.
2. Description of The Prior Art
Fluid shock absorbers have a wide range of uses. One such use is in damping relative movement of parts of a vehicle, such as an automobile. Typically, fluid shock absorbers are used to damp the oscillations of vehicle wheels relative to the vehicle frame. A shock absorber is generally used in conjunction with a spring, such as a coil spring or leaf spring, at each wheel.
Often, instead of a shock absorber/spring combination, a MacPherson strut is used. A MacPherson strut combines the functions of the shock absorber and the spring, and also serves as a structural link in the vehicle suspension. A MacPherson strut typically includes a hydraulic shock absorber, a coil spring surrounding the shock absorber, and means for attaching the strut o the vehicle frame and to the wheel. The shock absorber includes a cylinder, a piston movable in the cylinder and a piston rod attached to the piston and extending through an end of the cylinder.
In a MacPherson strut, the end of the shock absorber piston rod is the upper attachment point for the strut; that is, it is the part which attaches to the vehicle frame. Since the MacPherson strut serves as a structural link in the vehicle suspension, the strut is subjected to substantial radial forces, i.e., forces which act in a direction transverse to the longitudinal axis of the shock absorber. These forces are transmitted through the piston rod incorporated in the strut. Accordingly, a shock absorber which is used in a MacPherson strut must be designed to withstand substantial radial forces without impairing its operation.
A MacPherson strut is generally mounted in a near-vertical orientation, since most wheel travel is in the vertical direction. When a MacPherson strut is used in association with a front wheel in a vehicle, the maximum overall length of the MacPherson strut (strut extended) dictates the height of the upper strut mounting point and, thus, dictates the external hood height of the vehicle. Hood height reduction is desirable to improve vehicle aerodynamic efficiency. To reduce the hood height it is necessary, therefore, to reduce the maximum overall strut length.
The maximum overall strut length is determined generally by adding the cylinder stroke length, the piston rod free travel length, and the length of the piston rod/cylinder overlap. The cylinder stroke length is equal to the distance the piston travels along the cylinder from the point of full compression at one end to the point of full extension at the other end of the cylinder. The piston rod free travel length is equal to the distance the piston rod extends outwardly of the end of the cylinder when fully extended. The piston rod/cylinder overlap is equal to the length of the piston rod which is still within the cylinder when the shock absorber is fully extended.
The function of the piston rod/cylinder overlap in a shock absorber is to provide structural rigidity against radial deflection of its parts including the piston rod when acted upon by forces transverse to the piston rod. However, this overlap increases the maximum overall strut length. Other factors which contribute to structural rigidity and affect the maximum overall strut length include the cylinder end bearing thickness (length) and the piston/cylinder wall bearing length (effectively the piston thickness). Reduction of these lengths and of the amount of piston rod/cylinder overlap would desirably contribute to reduction of the maximum overall strut length, without changing the length of the cylinder stroke.